A rise in concern over terrorism has increased interest in real time detection and identification of aerosol particles, as radiological, chemical, biological, and explosive materials and their precursors can all be found in aerosolized form.
Recent advances in aerosol mass spectrometry enable real time analysis of aerosol particles in the field, critically important at high value terrorism targets, such as centers of government, airports, and sports venues. Aerosol mass spectrometry requires conditioning the aerosol under analysis, forming a particle beam, and analyzing individual particles by laser desorption/ionization mass spectrometry. Conditioning can include generating the aerosol, if the particles are not already aerosolized, and adjusting the concentration of particles. A particle beam is formed by the supersonic expansion of the gas surrounding the particles through focusing apertures. Particles in a liquid stream may similarly be formed into a beam. Individual particles in the particle beam are then analyzed by mass spectrometry.
Dense particle concentrations can adversely impact the ability of aerosol mass spectrometers to accurately track, resolve, and analyze individual particles. Inaccurate velocity measurement can skew the timing necessary for precise actuation of an ionizing laser. Further, resolving high particle density has historically taken increased complexity and expense.
For instance, the rapid single particle spectrometer (RSMS), disclosed in McKeown et al., “On-Line Single Particle Analysis by Laser Desorption Mass Spectromerty,” 63 Anal. Chem. 1906, 2069 (1991), the disclosure of which is incorporated by reference, utilizes a continuous wave laser for detecting the presence of a particle within an aerosol beam. Each particle scatters light while crossing a continuous laser beam. A photomultiplier tube (PMT) measures light scattering intensity to provide an approximate measure of particle size using the particle's reflective index, an inherently inaccurate size measurement. A pulsed laser is aimed adjacent to the continuous laser beam and is actuated upon detection of the particle. A mass spectrum is collected from material desorbed/ionized from the particle.
An aerosol time-of-flight mass spectrometer (ATOFMS), disclosed in U.S. Pat. No. 5,998,215, issued to Prather et al., the disclosure of which is incorporated by reference, sizes aerosol particles based on estimated velocity. Particle traversal time over the distance spanned by two continuous lasers is measured and extrapolated to trigger actuation of a mass spectrometer pulsed laser. Particle velocity yields more accurate particle size determination than does light scattering intensity, but the ability to track individual particles suffers as aerosol concentration increases, thereby limiting analytic throughput. Additionally, utilization of two lasers increases complexity over single laser designs.
An aerosol mass spectrometer system employing an array of six continuous lasers is disclosed in U.S. Pat. No. 7,260,483, issued to Gard et al., the disclosure of which is incorporated by reference. Throughput is improved by using a six-laser array to track the trajectories of individual aerosol particles. Higher performance at increased particle concentrations is achieved, but at the expense of size, cost, and complexity.